1. Field of the Invention
This invention relates to high pressure reciprocating plunger-type and piston-type pumps used in the petroleum industry, and more particularly, to a manifold for a front-discharge pump used in manifolding discharge valves positioned substantially coaxially with respect to the plungers in the pump. The invention also relates to a special nut used on the manifold.
2. Description of the Prior Art
It is common practice in the petroleum industry to employ high pressure plunger-type pumps in a variety of field operations relating to oil and gas wells, such as cementing, acidizing and fracturing, among others. An example of such a high pressure pump is the Halliburton Services HT-400 Horizontal Triplex Pump manufactured by Halliburton Services of Duncan, Okla., the assignee of the present invention. These pumps are frequently used in pumping two-phase slurries. Two-phase slurries are those in which solid particles (the "solid phase") are suspended in a liquid (the "liquid phase"). A problem with pumping such two-phase slurries is that the solid phase particles can separate out of the carrier liquid and can collect in valves, elbows, and in the fluid ends of the high pressure pumps in the system.
In the pumps, these particles tend to become packed ahead of the pump plunger or piston. This can result in sudden overpressurizing of the fluid in the pump with resulting damage to one or more of the plungers, connecting rods, crankshaft, fluid end, valves or other parts of the pump drive train.
Prior solutions to the overpressurizing problems are disclosed in U.S. Pat. No. 4,508,133 to Hamid and U.S. Pat. No. 4,520,837 to Cole et al., both assigned to the assignee of the present invention. These inventions comprise a protective cover assembly having a shear disc surrounded by an annular outer portion, mounted in a cylinder of the fluid end of the plunger-type high pressure pump. When a predetermined force is generated by the plunger in the cylinder, such as in an overpressure situation, the shear disc of the cover is sheared and is propelled outwardly against the plug. In turn, this forces the impact disc against an edge of a circular recess in the outer end of the retainer. The impact disc, in shearing against the recess edge, safely dissipates the kinetic energy of the shear disc, while the pressure in the cylinder vents to the atmosphere, avoiding damage to the fluid end of the pump, the plunger, connecting rod, crankshaft, etc. A problem with these devices is that the sheared disc is subjected to cyclic loading. Cyclic stress causes fatigue and premature failure of the disc around the thin arcuate wall may occur even at low pump pressures. Another problem is that the thin area around the arcuate portion does not leave much thickness for corrosion allowance, and thus may fail prematurely when corrosion is present. A further problem with previous apparatus is that the shear disc is expensive to fabricate, and machining may leave machine marks which act as stress risers and compound the fatigue problem already mentioned.
An apparatus which solves the fatigue problem is disclosed in U.S. Pat. No. 4,771,801 to Crump et al., also assigned to the assignee of the present invention. In this invention, a cover with a convex or domed center portion is used and is adapted for buckling away from the pump plunger when the pressure in the pump exceeds a predetermined level. The convex portion buckles when excessive force is transmitted from the plunger through any packed solid particles, allowing the packed particles to be pushed through vent passages.
Even though the apparatus of Crump et al. greatly reduces the risk of fatigue failure over Hamid and Cole et al., all three have two additional problems. First, once failure has occurred, the pump must be taken out of service immediately upon rupture of the protective cover. Secondly, a large quantity of the pump fluid can be spilled out of the pump when the cover ruptures. This limits the suitability of the apparatus for pumping some hazardous fluids. Also, the use of such shear discs obviously add somewhat to the operating costs of the pump in which they are employed.
In U.S. Pat. No. 5,073,096 to King et al., assigned to the assignee of the present invention, a pump fluid end is disclosed which solves the problems of the prior art by providing a pump in which a discharge valve is placed coaxially with the pump plunger or piston so that, if solid particles become packed ahead of the plunger, the force is transmitted through the particles to the discharge valve, pushing the valve open. This allows the packed particles to be pushed through the valve opening into the discharge passage. During this process, the pump may remain in use, and all fluids remain contained within the pump and its associated plumbing. The pressure containing envelope of the pump contains no designed "weak links" which present the possibility of failing and spilling fluid, as is possible in the other prior art.
A possible problem with using the apparatus of King et al. with conventional piping is that there may still be fluid flow problems in the discharge piping system. The present invention provides a front discharge manifold for the front discharge pump of King et al. The manifold is designed so that fluid flow therethrough will be smoother, thereby causing fewer pressure spikes. The reduction in such pressure spikes reduces the cyclic loading on the discharge valves and minimizes the fatigue resulting therefrom. The manifold also allows connection of multiple discharge valves even when the cylinders, and thus the discharge valves, are in close proximity to one another. By such a manifold, separate piping for each discharge valve is no longer necessary which reduce the costs and down time related to servicing.
Discharge valves which are close to one another may be manifolded because of a special nut which allows multiple connections to be made in a straight line while not interferring with nuts on adjacent cylinders. Basically, the invention allows more connections in a smaller space.